6 - Protein Synthesis and Tracking

Outline for Today

I. Protein SynthesisA. Overview of Information Transfer

B. Transcription

C. Translation

II. Protein TraffickingA. What goes where and why care?

B. Ribosome role

C. Pathway of secreted protein

Overview

Fig. 4.10

Overview

What is a gene?

• Gene = sequence of nucleotides that codes for the synthesis of a piece of RNA

• (simplified version)

http://library.thinkquest.org/19037/genome.html

RNA

• Ribonucleic acid• Types of RNA

– mRNA (messenger RNA)

– tRNA (transfer RNA)– rRNA (ribosomal RNA)

http://folding.stanford.edu/education/m.html

Overview

mRNA

• Patterned from DNA in the nucleus

• Moves to the cytoplasm

• 10% of RNA is of this type

• 3 base sequence constitutes a codon http://www.alumni.ca/~mcgo4s0/t3/RNA.html

Overview

tRNA

• Carries the amino acid from the cytoplasm to the ribosome

• Anticodon is a recognition area – Complimentary to the

codon of the mRNA

• Specific for an amino acid• 10% of the RNA is of this

type Fig. 4.8

Overview

rRNA

• The RNA portion of ribosomes• 80% of RNA is of this type

Fig. 4.9

Overview

Overview Protein Synthesis

Information transferfrom nucleus to cytoplasm

Culminates in protein formation

Stages

• Two stages to protein synthesis– Transcription– Translation

3rd. Ed Fig. 4.6

Overview

Stages

• Transcription– DNA to mRNA code

• Not all of the DNA is transcribed at once

– Language of the nucleotides

http://ghs.gresham.k12.or.us/science/ps/sci/ibbio/chem/nucleic/chpt15/transcription.gif

Overview

Stages

• Translation– Translate the code

from “A, C, G, and U” to amino acid language

– Nucleic acid language to amino acid language

3rd. Ed Fig. 4.6

Overview

Transcription

• DNA to mRNA– nucleus

• Stretch of DNA helix unwinds

• RNA polymerase• DNA rules of

complimentarity apply DNA RNA G C C G T A A U

http://ghs.gresham.k12.or.us/science/ps/sci/ibbio/chem/nucleic/chpt15/transcription.gif

Transcription

Transcription

Separation of strands

Enzymes link nucleotides together based upon laws of complementarity

Russo, handout

What are introns?(intervening regions)

• Consider the following

• inthebekweoitwoenasdfginninggodcreatedtheheavensdsjfaoiqwerjwqandtheearth

• inthebekweoitwoenasdfginninggodcreatedtheheavensdsjfa

oiqwerjwqandtheearth

• inthebeginninggodcreatedtheheavensandtheearth

mRNA Processing

http://ghs.gresham.k12.or.us/science/ps/sci/ibbio/chem/nucleic/chpt15/intron.htm

Don’t confuse introns and exons (expressed regions).

Finished mRNA

Trans-Trans link

Translation

Ribosome

Russo, handout

The Players!

Overview

• mRNA leaves the nucleus and goes to the cytoplasm

• Is recognized by the ribosomal subunit

Translation

Russo, handout

Overview

• Binding of subunits (amino acids) to get the polypeptide

Translation

Russo, handout

Role of tRNA

• “transfers” or carries the amino acid from the cytoplasm to the ribosome

• Anti-codon– 3 nucleotide base

sequence– Complimentary to the

codon

http://www.alumni.ca/~mcgo4s0/t3/RNA.html

Translation

Codon

• Triplet sequence on mRNA– 1 codon codes for 1

amino acid– Remember this was

coded from the DNA

Translation

Russo, handout

Anticodon

• Triplet sequence on tRNA– 1 anticodon for a

specific amino acid

Translation

Russo, handout

All RNA’s involved• rRNA makes up the ribosome• tRNA brings the amino acid to

the ribosome• mRNA carries the message

from the nucleus (DNA)

• The three types of RNA together carry out translation

http://www.cancerquest.org/index.cfm?page=45

Translation

Let’s Watch a Video

https://paris.mcgraw-hill.com/sites/0073525693/student_view0/chapter4/how_translation_works.html




Steps• Initiation

– tRNA with amino acid binds to ribosome subunit

– AUG is start codon• Therefore UAC is the anti-

codon– Other subunit comes in– Another tRNA comes in with

an aminio acid– Form the first peptide bond

– Ribosome shifts a distance of 1 codon

– How is the correct amino acid brought to the ribosome?

http://kvhs.nbed.nb.ca/gallant/biology/translation_initiation.jpg

Translation

The genetic code

Campbell et al., Fig. 10.11

This lets you determine what amino acid is brought to the ribosome.

(read in mRNA)

Translation

Steps

• Elongation– Ribosome moves

along the mRNA– Bond forms between 2

adjacent amino acids– Bond breaks between

amino acid and tRNA– Process keeps going

like a ticker tapehttp://www.science.siu.edu/microbiology/micr302/figure%207.25.JPG

Translation

Elongation = lengthening of peptide chain

Translation

Russo, handout

Steps

• Termination– The process stops

because the protein is complete

– Get one of the 3 stop codons• UAA• UAG• UGA


Translation

Termination = addition of amino acids stops

Stop Codon =Codon for which there is nocomplementary anticodon

Translation

Overview of the process


Translation

Relationships

Fig. 4.10

Protein Synthesis

Translation

Ribosome

= amino acid

mRNA message is “read” as triplet sequence of nucleotides: CODON

Mutations

• Mutation is a change in the base of the DNA– At least one type of mutation

is this type– (There are others that involve

an addition or a deletion of a base

• Sickle cell disease is caused by one change in the DNA bases resulting in an improper amino acid in the hemoglobin

• A mistake at either transcription or translation can result in problems


Protein Synthesis

Fig. 4.11

What goes where• Way of determining

what protein goes where in the cell– Tay-Sachs disease the

proteins are supposed to go to the lysosome but they don’t

– Get fatty acids accumulations

http://medgen.genetics.utah.edu/photographs/pages/tay.htm

http://www.dynagene.com/education/tay.html

Protein Trafficking

Ribosome location

– Free ribosomes• Protein will stay in the

cytoplasm or move to the nucleus or mitochondria

• Stays in the cell

– Anchored (to er)• Associated with the ER• Proteins destined for

the plasma membrane , the lysosome,or released from the cell (secretion)

Fig. 3.26b

Protein Trafficking

Fig. 3.26a

Pathway• Initial sequence of amino

acids is a signal– Called the signal sequence

• Signal is recognized by a particle in the cytoplasm– Signal recognition particle

(SRP)

• SRP pulls the complex to the endoplasmic reticulum– Signal peptide goes into

the lumen of the ER

Protein Trafficking

Fig. 4.11

Synthesis begins

RER

Initial sequence of amino acids is a

SIGNAL that directs protein to

the ER

“element” in cytoplasmrecognizes signal

and moveswhole complex to the

surface of the RER

Signal sequence istranslocated across

membrane of the ER to the inside

whole protein threadsinto the interior

of the ER

Protein Trafficking

Protein Trafficking• Inside the ER the signal

sequence is cleaved• Can be altered in the ER

– Remove some aa’s, folding, stabilizing with S-S bridges, adding carbs, etc.

– Insulin starts out as 1 chain of 86 aa’s but ends up as 2 chains of 21 and 30 aa’s.

• Protein put into a bud of the ER that forms a transport vesicle

• Transport vesicle goes to the Golgi– Fuses with Golgi– Protein is liberated into the

Golgi

Protein Trafficking

Fig. 4.11

Protein TraffickingRER

Signalis removed“budding”

Completed proteininside ER

Transport vesicle“shuttle”to Golgi

Fusion with Golgi

Golgi“modifications” occurthat provide sorting signals

“budding”

Secretory vesicleStorage vesicle

Fate #1

Immediate secretionRegulated secretion

Fate #2

Stimulus Applied

“shuttle”to membrane

Pathway

• In the Golgi it is modified and sorted

• Bud forms from the Golgi– Secretory vesicle– Leaves via exocytosis

Fig. 4.11

Protein Trafficking

Pathway

• Two possibilities for secretion– Immediate release– Regulated storage

• Vesicles stay in cell for a while as storage vesicles

• e.g. neurons and endocrine cells

Fig. 4.11

Protein Trafficking

Protein

Trafficking

Synthesis begins

DNA

mRNA

Free ribosomal subunits

in the cytoplasm

mRNA

Ribosome subunits

associate with mRNA

Signal

sequence

“element” in cytoplasm

recognizes signal

and moves

whole complex to the

surface of the RER

Signal sequence is

translocated across

membrane of the ER

to the inside

whole protein threads

into the interior

of the ER

RER

Initial sequence of

amino acids is a

SIGNAL that directs

protein to

the ER

Transport vesicle

RER

Golgi

An enzyme inside the RER

will cleave off

the signal

sequence

Protein is liberated into

the interior of the RER

once translation ends

The ribosome dissociates

from the mRNA

The signal piece will be degraded; the remainder of the protein will be

further processed

Eventually the protein

is moved into a membrane “bud”

which will generate a

transport vesicleThe transport

vesicle

shuttles the protein

to the Golgi

The transport vesicle

fuses with the Golgi and

releases the protein inside

After more processing, the protein

is moved into a membrane “bud”

which will generate a

secretory vesicle

Secretory vesicle

Secretory vesicleStorage vesicle

Fate of the Secretory Vesicle

In some cases the secretory vesicle

will transition to the plasma membrane

for immediate secretion of the contents

into the extracellular space

Plasma membranePlasma membrane

In some cells the vesicle can remain as a storage vesicle

for a period of time;

this is true in cells that have Regulated Secretion

Regulated Secretion requires that a stimulus

(trigger event) be applied to the cell to make the

storage vesicle release its contents

into the extracellular space

STIMULUS!

Regulated secretion Secretion into the extracellular space

6 - Protein Synthesis and Tracking

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